Peter Marks, who joined the Co-op as a 17-year-old supermarket shelf stacker, will step down as chief executive at the annual general meeting next May after the Lloyds deal has officially been cleared by the City's regulator, the Financial Services Authority.

Marks, 62, said his retirement had been planned for a long time but was kept secret until the Project Verde purchase – for an initial £350m with another £400m possible by 2027 – was signed to avoid any threat to the ambitious expansion of Co-op financial services. The deal will treble Co-op's high street banking estate to more than 1,000 branches with a 10% market share.

"Integration [of the banks] is going to take three to five years – that will take me to my late 60s," he said. "Having done the deal, it now feels like the right time to hand it over to my team."

He said the recent misbehaviour of the big banks has handed the Co-op, which is run and owned by its members, a wonderful opportunity to capitalise on the public's distrust of established lenders.

"Just look at the state the banking industry is in – Standard Chartered just today accused of money laundering and God knows what.

"The Co-op brand is back on track and in renaissance. And what a time to do it – mutualism is back in vogue what with what's happened in the plc world."

Marks, who oversaw the Co-op's £1.6bn takeover of the Somerfield supermarket chain in 2008 and the merger of the banking arm of the business with Britannia building society a year later, said: "It's been full-on for the last six years, my wife has been long-suffering. I'm going to relax a bit now and spend more time with her and I've got a grandchild now."

"I've been here since leaving school at 17 – I've had a wonderful career," he said. "I started very much at the bottom filling shelves at the supermarket, now I'm running one of the biggest businesses in the UK.

"It's a funny feeling. Even though I've known for a while that I'm leaving, having spent almost 45 years – almost forever – here it's a wrench to leave. But you've got to make way for the younger generation."

Marks, who served as Co-op's chief executive for the last six years, said the board has begun to search for his successor.

The Co-op's chairman, Len Wardle, said: "Peter has done a truly outstanding job for the Co-operative Group. He was the architect of the current strategy to ensure that we developed real scale in our key businesses."

Marks will continue his role as a non-executive director of Thomas Cook and look for other boardroom positions.

He joined Yorkshire Co-operatives in 1967 as a management trainee and rose to become chief executive in 2000. He then became chief executive of the enlarged Co-operative Group in 2007 when United Co-operatives merged with the Co-op Group.

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